Personal air sampling devices can be used to determine the exposure of individuals to various airborne particles and contaminants over a period of time. For example, U.S. Pat. No. 4,721,517 to Cloutier and U.S. Pat. No. 5,119,682 to Bellinger each describe sampling devices configured to be worn by an individual and to obtain air samples within the breathing zone of the wearer. Sampled air is pumped through a filter which retains any airborne particles and/or contaminants. Subsequently, the filter can be weighed or subjected to other types of analysis in order to determine the nature of the collected particles and/or contaminants.
Air flow requirements for air sampling are conventionally based on the expected concentration of airborne particles, and the ability to measure and analyze the particles collected. Collection of too much particulate material can clog a filter and can cause a sampling flow rate to drop below acceptable levels. Sampling pumps for personal sampling systems conventionally operate at a fixed flow rate and with size-selective sampling inlets. Sampling periods are conventionally based upon the amount of particulate material collected. For example, shorter sampling periods may be used within environments having a higher average concentration of airborne particles.
In order to reduce inconvenience to a wearer, conventional personal air sampling devices typically are designed to be as small and as lightweight as possible. To facilitate portability, personal air sampling devices are conventionally battery powered. However, batteries may have a limited lifetime which can result in sampling periods that are not long enough to obtain a satisfactory air sample. In order to extend air sampling periods, intermittent air sampling can be performed wherein a sampling pump is operated intermittently, creating "ON" periods and "OFF" periods. (See, for example, U.S. Pat. No. 5,201,231 to Smith and U.S. Pat. No. 4,080,832 to Moody et al.). Intermittent air sampling can extend battery life so that air sampling can be extended over a longer time period than achievable via continuous air sampling. Intermittent air sampling also facilitates obtaining representative aerosol samples over an extended period of time without "blinding" a filter by collecting too much particulate matter.
It would be valuable to independently verify that an air sampling device was worn by an individual during a time period designated for sampling (i.e., an ON period during intermittent operation). Unfortunately, conventional personal air sampling devices designed for intermittent operation may not provide an accurate way of ensuring that an individual is wearing a device during an ON period. Also, it would be valuable to know the level of physical activity of the wearer of an air sampling device during sampling because an inhaled dose of airborne particles may be a function of breathing rate as well as of sampling time.
It is known to sample air in proportion to the actual respiration of an individual. For example, in U.S. Pat. No. 4,183,247 to Allen et al., a sampling pump is activated by the expansion and contraction of an individual's thoracic cavity during respiration. In addition, it is known to sample air in proportion to the pulmonary output of an individual. For example, U.S. Pat. No. 4,589,292 to Delhaye et al. describes a process and device for sampling ambient air in proximity of the respiratory track of an individual wherein a sampling pump is piloted by a signal indicating the individual's cardiac rhythm, such as from the pulsed flow of blood in an ear lobe of the individual. Unfortunately, existing methods of monitoring physical activity of a person are somewhat personally intrusive and complex.